86 THE ORIGIN OF VERTEBRATES 



thickening of the cnticnlar layer to form a lens could only exist as 

 long as that layer is absolutely external, so that the light strikes 

 immediately upon it ; for, if from any cause the eye became situated 

 internally, the place of such a lens must be filled by the structures 

 situated between it and the surface, and the thickened cuticle would 

 no longer lie formed. 



In all vertebrates these pineal eyes are separated from the 

 external surface by a greater or less thickness of tissues ; in the 

 case of Ammoccetes, as is seen in Fig. 31, the eye lies within the 

 membranous cranial wall, and is attached closely to it. The position, 

 then, of the cuticular, or corneal lens, as it is often called, on the 

 supposition that this is a median eye of the arachnid type, is taken 

 by the membranous cranium, and, as I have described in my 

 paper in the Quarterly Journal, on carefully lifting the eye in the 

 fresh condition from the cranial wall, it can be seen under a 

 dissecting microscope that the cranial wall often forms at this 

 spot a lens-like bulging, which fits the shallow concavity of the 

 surface of the eye, and requires some little force to separate it from 

 the eye. 



As will appear in a subsequent chapter, this cranial wall has 

 been formed by the growth, laterally and dorsally, of a skeletal 

 structure known by the name of the plastron. The last part of it to 

 be completed would be that part in the mid-dorsal line, where appa- 

 rently, in consequence of the insinking of the degenerating eyes, a 

 dermal and subdermal layer already intervened between the source 

 of light and the eyes themselves. 



When the membranous cranium was completed in the mid-dorsal 

 region, it was situated here as elsewhere just internally to the sub- 

 dermal layer, and therefore enclosed the pineal eyes. This, to my 

 mind, is the reason why the pineal eyes, which, in all other respects, 

 conform to the type of the median eyes of an arachnid-like animal, 

 do not possess a cuticular lens. Other observers have attempted to 

 make a lens out of the elongated cells of the anterior wall of the 

 eye (my corneagen layer), but without success. 



Studnicka, who calls this layer the pellucida, does not look upon 

 it as the lens, but considers, strangely enough, that the translucent 

 appearances at the ends of each nerve end-cell represent a lens for 

 that cell, so that every nerve end-cell has its own lens. Still more 

 strange is it that, holding this view, he should yet consider these knobs 



